The present invention was originally developed in connection with hydrocracking of a heavy hydrocarbon feedstock high in content of asphaltenes and sulfur moieties. More particularly, the feedstock tested was vacuum tower bottoms ("VTB") produced from distillation of bitumen. The invention is not limited in application to such a feedstock; however, it will be described below with specific respect to it, to highlight the problems that required solution.
Bitumen contains a relatively high proportion of asphaltenes. When the bitumen or its vacuum tower bottoms are hydrocracked, the asphaltenes produce coke precursors, from which adherent solid coke evolves. The coke deposits on and adheres to the surfaces of the reactor and downstream equipment. In addition, since part of the feedstock is consumed in the production of coke, the conversion of the feedstock to useful products is reduced.
The present assignee is an Alberta government research agency which was given a mandate to foster improvements in the upgrading of bitumen and other heavy oils. Realizing the conversion limitation and operating problems that coke deposition inflicts, it initiated a research project to investigate the mechanisms of coke formation and to look for improvements that might be applied commercially.
The present processes were generated as a result of this work. The research involved a progression of concepts and experimental discoveries that came together to yield a process characterized by a high order of conversion coupled with reduced deposition of adhesive coke and reduced production of coke.
Searches and prosecution of the parents of this application have identified the following relevant prior art:
U.S. Pat. No. 4,294,686 (Fisher et al) teaches that, when liquid hydrogen donor oil is used along with hydrogenation in connection with hydrocracking of bitumen vacuum tower residua, coke deposition is allegedly eliminated.
However the present assignee and the assignee of the above cited patent jointly conducted a large scale hydrocracking test on bitumen residue using a liquid hydrogen donor process. This test encountered serious coke production problems. It appears that hydrocracking high asphaltene content feed such as bitumen residue requires more than the presence of liquid hydrogen donor oil alone.
U.S. Pat. No. 4,455,218 (Dymock et al) teaches use of Fe(CO).sub.5 as a source of catalyst formed in situ for hydrocracking heavy oil in the presence of H.sub.2. The reaction is allegedly characterized by elimination of coking.
U.S. Pat. No. 4,485,004 (Fisher et al) teaches hydrocracking heavy oil in the presence of hydrogen, hydrogen donor material, and catalyst comprising particulate Ni or Co on alumina.
U.S. Pat. No. 4,134,825 (Bearden et al) teaches forming solid, non-colloidal catalyst in situ in heavy oil using trace amounts of Fe added in the form of an oil-soluble compound such as iron carbonyl. The metal compound is added to the oil and heated to 325.degree.-415.degree. C. in contact with hydrogen to convert it to a solid, non-colloidal, catalytic form. This catalyst is then used in hydrocracking the oil and it is stated that coke formation is inhibited.
U.S. Pat. No. 4,592,827 (Galliasso et al) teaches injecting an oil-soluble, catalyst precursor Mo compound and water into a heavy oil stream moving to a heater, wherein the mixture is heated to a temperature of 230.degree. C.-420.degree. C. to effect decomposition of the Mo compound. The heater product is then introduced into a hydrocracking reactor.